The present invention relates to an analog optical transmission apparatus for optical fiber communication and, more particularly, to an analog optical transmission apparatus suitable for SCM (Sub Carrier Multiplex) transfer.
In a CATV (CAble TeleVision) system or the like, an SCM scheme is used for data transfer. When data transfer based on the SCM scheme is used for optical transfer, an intermodulation distortion due to the nonlinearity of the I-L (current vs. optical output) characteristics of a semiconductor laser often poses a problem.
An analog optical transmission apparatus having a function of compensating for such an intermodulation distortion is disclosed in Japanese Patent Laid-Open No. 5-27582 (reference 1).
FIG. 2 shows the schematic arrangement of the analog optical transmission apparatus disclosed in reference 1. Referring to FIG. 2, reference numeral 11 denotes an analog signal source; 12, a semiconductor laser connected to the analog signal source 11; 13, an optical fiber for transmitting an optical signal output from the semiconductor laser 12; 14, an optical fiber coupler optically coupled to the optical fiber 13; 15, a light-receiving element for receiving the output light from the optical fiber coupler and converting the light into an electrical signal; 16, an amplifier for amplifying the output signal from the light-receiving element 15; 17, a delay line for delaying the output from the analog signal source; and 18, a differential amplifier for differentially amplifying the output from the amplifier 16 and the output from the analog signal source 11 through the delay line.
The semiconductor laser 12 is intensity-modulated by the analog signal source 11 and outputs an optical signal to the optical fiber 13. At this time, part of the optical signal passing through the optical fiber 13 is extracted by the optical fiber coupler 14. The optical signal extracted by the optical fiber coupler 14 is converted into an electrical signal by the light-receiving element 15. The electrical signal converted by the light-receiving element 15 contains an intermodulation distortion (cross modulation distortion) generated by the semiconductor laser 12. The electrical signal containing the distortion component is amplified by the amplifier 16 and input to the first input terminal of the differential amplifier 18. The signal from the analog signal source 11 is input to the second input terminal of the differential amplifier 18 through the delay line 17.
When the amplitude and phase of the signal output from the analog signal source 11 match those of the signal output from the light-receiving element 15, the carrier wave (carrier) contained in the signal is canceled as a common mode signal of the differential amplifier 18, so only the distortion component is extracted. At this time, the delay line 17 adjusts the phase of the signal output from the analog signal source 11 and the phase of the signal output from the light-receiving element 15 such that the two phases match. The signal corresponding to only the distortion component, which is output from the differential amplifier 18, is superposed on the signal to be output from the analog signal source 11 to the semiconductor laser 12. When the amplitude and phase of the distortion generated by the semiconductor laser 12 are inverted to those of the distortion component output from the differential amplifier 18, the distortion can be canceled.
However, in the conventional analog optical transmission apparatus, distortion compensation in a broadband is impossible in principle. More specifically, in the analog optical transmission apparatus shown in FIG. 2, the light emitted by the semiconductor laser 12 is monitored by the optical fiber coupler 14 inserted midway in the optical fiber 13 and by the light-receiving element 15. The distortion component generated by the semiconductor laser 12, which is detected upon monitoring, is superposed on the signal from the analog signal source 11 to the semiconductor laser 12. At this time, a time lag occurs between the timing when the distortion is generated by the semiconductor laser 12 and the timing when the distortion component is superposed on the signal output from the analog signal source 11 to the semiconductor laser 12. That is, the phases of the distortion components cannot be made to match in principle. In this case, the distortion cannot be compensated for in a broadband.